Chapter round hole. It is made possible by

 

 

 

 

Chapter 1

 

Introduction

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1.1
Background

Machining is a very important process of
manufacturing by which jobs are produced to the required dimensions and surface
finish by slowly removing the excess material from the workpiece in the form of
chips with the help of cutting tool(s) moved past the work surface(s). The
essential basic requirements for machining work are

                  

Figure 1.1 Schematic representation of
machining process to end product

 

It is mostly used for
metal products, but also used for materials like wood, plastic, ceramic and
composites. The processes that have this common
theme, controlled material removal, are today collectively known as subtractive manufacturing,

Parts manufactured by
shaping, forming, shaping process and etc often require to be machined before
the product is usable. Machining technology has produced products with desired
features like smooth shiny surfaces, small-diameter deep holes, threaded holes
etc. The history in machining ranges back to the 18th century where tools made
of hardened carbon steel were used to machine easy to cut materials like
bronze, brass and gray cast iron. The technology has progressed ever since and
advanced machining technology is available in the 21st century where machining
hard materials like titanium, ceramics etc is done by coated carbide tools.

1.1.1 Machining operations

There are many kinds of machining operations
where each is capable of generating a certain part geometry and surface
texture.

During turning operation, the cutting tool
with single cutting edge is utilized to subtract material from a workpiece
rotating in order to generate a cylindrical shaped part. The primary motion is
provided by rotating the workpiece, and the feed motion is achieved by moving
the cutting tool slowly in a direction parallel to the axis of rotation of the
workpiece.

Figure 1.2 Turning
Operation

Drilling is used to create a round hole. It is
made possible by a rotating tool that typically has two to four helical cutting
edges. The tool is fed in a direction parallel to its axis of rotation into the
workpiece to form the round hole.

Figure 1.3 Drilling
Operation

During boring operation, a tool with single bent
pointed tip is put into a roughly made hole in a spinning workpiece to slightly
increase the size of the hole and improve the accuracy of hole dimensions. It
is a kind of fine finishing operation used in the final stages of product
manufacture.

Figure 1.4 Boring Operation

Reaming is type of sizing operations that
subtracts a small amount of metal from a hole that is already drilled.

Figure 1.5 Reaming
Operation

In milling, a rotating tool with multiple cutting edges
is moved slowly relative to the material to generate a plane or straight
surface. The direction of  feed motion is
always perpendicular to the tool’s axis of rotation.The rotating milling cutter
provides the speed motion..

Figure 1.5 Milling
Operation

1.1.2
Cutting Tool

A cutting tool has one or more than one sharp cutting
edges capable of separating the chip from parent workpiece. The cutting
tool is always harder than the workpiece.

There are two surfaces in the cutting tool:

1.Flank

The flank of the tool gives a clearance
between tool and the newly formed work surface, which protects the surface from
abrasion, which would likely degrade the finish. This angle between the work
surface and the flank surface is called the relief angle.

2.Rake face

The rake face directs the flow of newly
formed chip,which is oriented at a certain angle is called the rake angle
“?”. It is measured relative to the plane and perpendicular to the
work piece surface . The rake angle can be positive or negative.There are two
basic types of cutting tools:

·        
Single
point tool

A single point tool has one cutting edge and
is used for turning, boring and planing . While machining, the point of the
tool penetrates below the original work surface of the workpiece. The point is usually
rounded to a certain radius called nose radius.

 

·        
Multiple
cutting edge tool

Multiple-cutting-edge tools have more than
one cutting edge and usually achieve their motion relative to the workpiece by
rotating. Drilling and milling uses rotating multiple-cutting-edge tools.
Although the shapes of these tools differ from the single-point tool, many
elements of tool the geometry are very similar.

 

Factors affecting tool life:

Tool life is affected by
many factors such as cutting speed, depth of cut, chip thickness, tool
geometry, material or the cutting fluid and rigidity of machine. Physical as
well as chemical properties of work material influence tool life by affecting
form stability and rate of wear of tools The nose radius also tends to affect
tool life.

 Cutting speed

Cutting speed has a major influence
on tool life. As the cutting speed increases the temperature also increases.
The heat is more concentrated on the tool than on the workpiece and the
hardness of the tool metrix changes so the relative increase in the hardness
of the work accelerates the abrasive action. The criterion of wear depends on
the cutting speed because the predominant wear may be wear of flank or crater
if cutting speed increases.

 Feed and depth of cut

The feed rate influences
tool life also. With fine feed the area of chip passing over the tool face is
more than that of coarse feed for a given volume of swarf removed, but to
offset this chip will be greater hence the resultant pressure will nullify
the advantage.

Tool Geometry

Tool geometry affects tool
life also.. Tool with a large rake angle becomes weak as a large rake angle reduces
the tool cross-section and the amount of metal to absorb the heat.

Tool Material

Physical as well as
chemical properties of work material influence tool life by affecting form
stability and rate of wear of tool.

Cutting Fluid

 Cutting fluids reduce the coefficient of friction at the
chip tool interface and also increase the tool life.

Table 1.1:Properties affecting Tool Life

 

1.2
Cutting Forces

 

Cutting is a process of extensive
stresses and plastic deformations. The high compressive and frictional contact
stresses on the tool face result in a substantial cutting force F.

Knowledge in cutting forces is
essential for the following important reasons:

1. Proper design of the cutting tools

2. Proper design of the fixtures used
to hold the workpiece and cutting tool

3. Calculation of the machine tool
power

4. Selection of the cutting conditions
to avoid an excessive distortion of the             workpiece.

Figure 1.6 Cutting Force Diagram

 

1.2.1
Cutting force components In orthogonal cutting

 The total cutting
force F is resolved into two components in the horizontal and vertical path,
which can be measured directly using a force measuring device called dynamometer.

Total cutting force F is conveniently resolved into
horizontal component FC and vertical component FD The two force components act
against the tool: ?

Cutting force FC : this force is in the direction of
primary motion. The cutting force constitutes about 70~80 % of the total force
F and is used to calculate the power P required to perform the machining
operation, P = VFC 

 Thrust force FD:
this force is in direction of feed motion in orthogonal cutting. It is used to
calculate the power of feed motion.

In  3-D oblique
cutting, one more force component appears along the third axis. The thrust
force FD is further resolved into two more components, one in the direction of
feed motion called feed force Ff , and the other perpendicular to it and to the
cutting force FC called back force Fp , which is in the direction of the
cutting tool axis.

1.2.2
General methods of measurement of cutting forces

There are generally two ways
through which cutting forces are measured one is indirectly and the other is
directly that it is using tool force dynamometers. In the indirect method
cutting forces are measured through cutting power consumption and calorimeter
method. This method gives inaccurate readings and only gives an average
estimate of the forces. Whereas in the second method that is using tool force
dynamometers they give accurate and precise results moreover they are versatile
and reliable.

1.2.3
Cutting force control

The cutting force value is primarily affected by: v
cutting conditions (cutting speed V, feed f, depth of cut d) ,cutting tool
geometry (tool orthogonal rake angle) , properties of work material. The
simplest way to control cutting forces is to change the cutting conditions.

Figure
1.7 Cutting force parameters relations(Graphically)

 

 

1.3
Tool Geometry

Several angles are important when introducing the cutting
tool’s edge into a rotating workpiece.These angles include: the angle of
inclination , rake angle, effective rake angle,  lead or entry angle , tool nose radius. The
angle of inclination when viewed from the side or front is the angle of the
insert seat or pocket in the tool holder, from front to back. This inclination
can be either positive, negative, or neutral. The cutting tool’s rake angle is
the angle between the cutting edge and the cut itself. It may also be positive,
negative, or neutral. The effective rake angle is the combination of the tool
holder’s angle of inclination and the rake built into the insert. The lead or
entry angle is the angle between the direction of the cutting tool feed and the
cutting edge. The tool nose radius is the angle formed by the point of the
tool. This radius may be large for strength, or sharp for fine radius turning.
Since a sharp edge is weak and fractures easily, an insert’s cutting edge is
prepared with particular shapes to strengthen it. Those shapes include a honed
radius, a chamfer, a land, or a combination of the three.

Figure
1.8: Tool Geometry of single point cutting tool

 (i) Back rake angle:
Back
rake angle is the angle between the face of the single point cutting tool and a
line parallel with base of the tool measured in a perpendicular plane through
the side cutting edge. If the slope face is faced downward towards the nose, it
is called negative back rake angle and if the slope face is upward towards the
nose, it is positive back rake angle. Back rake angle helps in removing the
chips away from the workpiece.

(ii)Side
rake angle:
Side
rake angle is the angle in which the face of the tool is inclined side ways. It
 is the angle between the surface of the
flank immediately below the point and the line down from the point
perpendicular to the base. Side rake angle of cutting tool determines the
thickness of the tool behind the cutting edge. It is provided on the tool to
provide a clearance between workpiece and tool so that it prevents rubbing of
workpiece with end flake of tool.

 

 (iii)End relief angle:
End
relief angle is defined as the angle between the portion of the end flank
immediately below the cutting edge and a line perpendicular to the base of the
tool, measured at right angles to the flank.Rubbing is avoided on workpiece
with the help of end relief angle on the tool..

(iv)Side
relief angle:
Side
rake angle is the angle between the portion of the side flank immediately below
the side edge and a line perpendicular to the base of the tool measured at
right angles to the side. It is also the angle that prevents interference as
the tool enters the workpart. It is incorporated on the tool to provide a relief
between the flank and workpiece surface.

(v)End
cutting edge angle:
It is
the angle between the end cutting edge and a line perpendicular to the shank of
tool. It provides clearance between tool cutting edge and workpiece.

(vi)Side
cutting edge angle:
Side
cutting edge angle is the angle between straight cutting edge on the side of
tool and the side of the shank. It is responsible for turning the chip away
from the finished surface.

 

1.4
Heat Generation in Machining

Heat has critical influences on machining.
As it can increase tool wear and
then reduce tool life and
also give rise to thermal
deformation and cause to environmental problems. But due to the complexity of machining mechanics, it’s hard to
predict the intensity and distribution of the heat sources in an individual machining operation because the properties of materials used in machining vary with temperature, mechanical process and  thermal dynamic process which are
very tightly coupled together.
Since this century, many efforts in theoretical analyses and experiments have
been made to understand this phenomena, but many unsolved problems remain.

 

 

 

 

1.4.1 Heat Generated in Various Machining
Operations

Most
of the heat generation models were established under orthogonal cutting
condition. But in practice, there are various machining operations which cannot
satisfy this condition, such as oblique turnning, boring, drilling, milling,
grinding, etc.

Generally,
the intensity of heat sources in real life machining operations can be
determined by the external work applied in an approximate manner, but, the
distribution of the heat sources are hard to obtain by either theoretical or
experimental methods.

The
following listed are the simplified heat source model in real operations:

o    Boring: A uniform moving ring heat source.

o    End Milling: An ellipsoidal shape distribution with a
distribution of uniform heat flux at milling area.(Heat source not defined by
its intensity)

o    Grinding: A circular heat source moving on the surface
of workpiece.

1.4.2 Types of Heat Sources

There
are several types of heat source in machining:

Plastic
work converted to heat. Viscous dissipation is transformed into heat if the cut
material is visco-plastic. Work done by friction converted to heat. Ambient
heat source sometimes need be considered if thermal deformation is concerned.In
non-traditional machining, other types of heat sources exist.

1.4.3 Heat Generated in Primary Zone

Heat
generated in this zone is primarily due to plastic deformation as well as viscous
dissipation. But in classical machining theory, the rate of heat generated is
the product of the shear plane component, Fs, of the resultant force and the
shear velocity, Vs, i.e., the shear energy is completedly converted into heat.

1.4.4 Heat Generated in Secondary Zone

In
this region, because of the complexity of plastic deformation, this part of
heat was ignored in many prevoius theoretical research.

Boothroyd
has shown that the secondary plastic zone is roughly triangular in shape and that
strain rate, E.

Hence
the maximum intensity of heat source in this zone is proportional to the strain
rate.

1.4.5 Heat Generated at Interface between Tool
& Chip

Heat
is generated at the tool/chip interface by friction. The intensity,Ic, of the
frictional heat source is approximatedly by

F Vx

Ic = ————

h b

where
F is friction force, Vx is the sliding velocity of the chip along the interface,
and h is plastic contact length.

Figure
1.9 Sources of Heat in Metal Cutting

 

 

 

 

1.5 Cutting
fluids in machining

Cutting fluid is a type of coolant and lubricant designed specifically for metalworking processes, such as machining, stamping etc. There are various kinds of
cutting fluids, which include oils, oil-water emulsions, pastes, gels, aerosols (mists), and air or
other types of gases. They may be made from petroleum distillates, animal fats, plant oils, water and air, or other raw ingredients.
Depending on use and on which type of cutting fluid is being considered, it may
be called as cutting fluid, cutting compound, cutting oil, coolant, or lubricant.

Most metalworking and machining processes can
be benefited from the use of cutting fluid, depending on the workpiece
material. The most common exceptions to this are cast iron and brass, which is machined dry.

The properties that are sought after in a good
cutting fluid are the ability to:

·        
keep
the workpiece at stable temperature (critical when working to close tolerances). Warm is acceptable, but
extremely hot or alternating hot and cold are mostly avoided.

·        
They
maximize the life of cutting tip by lubricating the working edge and
reducing tip welding.

·        
They
ensure safety for the people handling it (toxicity, bacteria, fungi) and for
the environment upon disposal.

·        
They
also prevent rust on machine part as well as cutters.

1.5.1 Function of cutting
Fluids

There are mainly two functions done by
cutting fluids one is to cool the workpiece and the other is to bring about
lubrication between the workpiece and tool. The cooling of work piece is needed
during machining as the temperature of the workpiece increases enormously which
causes distortions to the workpiece structure and may also cause damage to the
tool structure so in order to reduce this temperature coolant/cutting fluids
are utilized. This heat also causes thermal expansion and oxidation which can
be avoided using cutting fluids. Besides cooling by lubricating the incoming
friction between the workpiece and tool can be avoided to a large extent by
lubrication.They are also useful in removing unwanted chips from the workpiece
during machining process from the cutting zone. They also increase tool life
and brings about corrosion resistance in the machined surface.

 

 

1.5.2
Properties of Cutting Fluids

Cutting
fluids should have a low viscosity to allow free flow of the fluid as well as good
lubricating properties. It should have high specific heat, high heat
conductivity and high heat transfer coefficient. It must non-corrosive to work
and machine. It should be odourless and must be  stable in use and storage.It should permit
clear view of the work operation.

 

 

1.5.3
Types of Cutting Fluids

There are various types of cutting fluids used in
practice. Nowadays cutting fluids are generally pastes/gels, liquids, air(or
other gases), CO2 coolants,aersols(mist).Each of them were used for different
purposes and in different ways.